How to Solve EMI Problems in Isolated Automotive Systems
Skyworks
This Silicon Labs whitepaper by Charlie Ice addresses EMI challenges in isolated automotive systems, particularly relevant to EVs and HEVs operating under the CISPR 25 standard. As these vehicles rely heavily on power electronics bridging high- and low-voltage domains, digital isolators — while sophisticated — introduce new EMI risks through common-mode currents and high-frequency RF signaling. The paper covers three no-cost techniques to reduce EMI. First, choose a digital isolator whose default output state matches the signal's idle state, minimizing how often the internal RF transmitter fires. Second, select bypass capacitors with low effective series inductance (ESL) — smaller or reverse-geometry packages — to keep the self-resonant frequency high and AC noise suppression effective. Third, place those capacitors as close as possible to both the supply and ground pins of the isolator to minimize loop area and trace inductance. Beyond these three methods, the paper briefly touches on Y capacitors across the isolation barrier as a further measure for stubborn common-mode current issues.